Romain Grosjean is sure Lotus will produce a better car in 2015 but is keeping his options for next season open.

After a breakthrough year in 2013, Grosjean has not had the car to compete at the front this season. He has made clear that he wants a winning car in 2015, but says that does not rule out Lotus, which is expected to switch to Mercedes engines in a bid to boost performance.

"I am 99.99% confident that the team is going to be better next year, not even speaking about the engine, just because it has been a transition year," Grosjean said. "It has been difficult and we were starting on the back foot and next year everything's going to be in a better shape. So even removing any power unit questions it's going to be better.

"If you look at Jenson [Button] in 2008 he was very much struggling with Honda and then changed to Brawn, which was the same team with a different name, and he was world champion. It's the same for [Fernando] Alonso, he had very good years and he had difficult years, and it's part of your career, you have to accept that and not get too frustrated.

"Sometimes it's hard because you are a competitor and you want to win races, fight at the front and you want things to go better. Sometimes they don't go better but you accept that it is going to be tough and go from there."

Grosjean said he would use the summer break to consider where he wants to drive next year.

"I think it's always in the back of your mind. I want to win races in the near future and we are working on that. Of course you think about your future … it's going to certainly be [something I think about] in the summer break."